This invention relates to nuclear reactors and, more particularly, to a reactor-core isolation cooling (RCIC) system. A major objective of the present invention is to provide for an RCIC system with enhanced effectiveness in the event of a loss of electrical power to the incorporating reactor plant.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
Dual-phase reactors store heat generated by the core primarily in the form of vapor pressure generated by the vaporizing of a liquid heat transfer medium. The vapor pressure can used to rotate a turbine that drives a power output generator to produce electricity. Condensate from the turbine can be returned to the reactor, merging with recirculating liquid for further heat transfer and cooling. The primary example of a dual-phase reactor is a boiling-water reactor (BWR). Dual-phase reactors are contrasted with single-phase reactors, which store energy primarily in the form of elevated temperatures of a liquid heat-transfer medium, such as liquid metal. The following discussion relating to BWRs is readily generalizable to other forms of dual-phase reactors.
Modern nuclear reactor plants are designed to handle a wide range of failure scenarios, such as those that might be induced by an earthquake. For example, reactors must be designed to handle an abrupt isolation of the turbine from the reactor by shutting down the reactor safely, while protecting the reactor core from damage due to overheating. Control rods can be inserted into the core to decrease its reactivity. Nonetheless, the core continues to generate a considerable amount of "decay" heat. In the absence of protective systems, the decay heat could create a pressure buildup within the reactor vessel. The pressure could cause a breach in the reactor vessel or in associated conduits. The breach could cause a loss of coolant. The loss of coolant could prevent transfer of heat from the core, which could then melt. This would, in essence, render the plant unrecoverable.
RCIC systems constitute one class of protective systems utilized in BWR reactors to protect the core in the event the main turbine is isolated from the reactor. An RCIC system regulates water level within a reactor pressure vessel by pumping water from an external reservoir into the vessel when the level falls below a predetermined threshold. The RCIC pump is driven by a RCIC turbine. Steam output from the reactor pressure vessel is diverted from the main steam line (which feeds the main turbine) to drive the RCIC turbine.
While the RCIC turbine itself is powered by steam, a typical RCIC system is dependent on electrical power. For example, RCIC operation requires that steam and water flows be rerouted, typically by opening and closing certain electrically operated valves. In addition, electricity is required by the control loop that regulates the rate at which water is pumped by the RCIC system into the reactor pressure vessel. In particular, this loop typically includes a flow element used to measure the RCIC pump output, control electronics required to compare a measured flow with a target flow, and an electrically-controlled hydraulically-powered turbine governor valve. Additional RCIC control electronics also require electricity for their operation.
Furthermore, some RCIC components, for example, the control electronics and motorized valves, are vulnerable to excessive heat. Heat accumulates due to conduction, radiation and convection from the steam driving the RCIC turbine and from dissipation from electrical circuits. To protect heat-sensitive RCIC components, cooling can be provided by a plant heating, ventilation and air conditioning (HVAC) system, which is typically AC powered.
Since a large number of RCIC system and other safety systems rely to some extent on electrical power, a safe reactor plant must address the scenario of a loss of AC power along with a shutdown of the main turbine. An earthquake could knock down power lines to a reactor plant and cause or force isolation of the main turbine, causing a station blackout. Thus, an RCIC operation must be available to handle decay heat despite a lack of electrical power from the main turbine and from an external electrical power grid and onsite diesel generators.
A nuclear reactor plant typically includes a large back-up battery in case the plant is decoupled from AC power. During normal operation, the battery is charged by the AC power. During a blackout, the battery replaces the AC power. However, the energy stored by such a battery is necessarily limited. Furthermore, the RCIC system must compete with many other plant systems for battery power. Typically, batteries can provide a few hours of electrical power. However, a reasonable target value would be for about eight hours of backup power. Greater sizes and numbers of batteries can be used to supply this additional power. However, there are problems with the cost, volume and complexity of a system with the necessary battery power. In addition, there is a problem in allocating battery power, since less important devices might drain the battery at the expense of the RCIC system, which is essential to protecting the reactor core during a blackout.
What is needed is an RCIC system which can provide for extended handling of decay heat and which has access to power relatively independent of demands of other plant subsystems. Preferably, this RCIC system would not require additional large batteries, or alternatives which are comparably expensive and voluminous.